WDM transmission is practically used as one technology that implements long-distance and large-capacity optical communication. In WDM, a plurality of optical signals with different wavelengths can be transmitted. In recent years, an optical transmission system has been proposed that transmits a WDM signal in which a plurality of optical signals generated in different transmission methods (generated in different modulation methods and/or transmitted at different bit rates) is multiplexed.
Many optical transmission apparatuses that transmit a WDM signal control all optical signals in different wavelength channels in the WDM signal so that the optical signals have the same transmission power. That is, the transmission power of a plurality of optical signals in the WDM signal is equalized. Systems in which optical signals transmitted at different rates or generated in different transmission methods are multiplexed are described in, for example, Japanese Laid-open Patent Publication No. 2001-333016 and Japanese Laid-open Patent Publication No. 2007-235412.
As described above, optical transmission systems are proposed that transmit a WDM signal in which optical signals generated in different transmission methods are multiplexed. However, strength to withstand quality deterioration depends on the transmission method. For example, the withstand strength for an optical signal-to-noise ratio (OSNR) in modulation methods in which many bits are transmitted with one symbol (such as 16-quadrature amplitude modulation (16QAM) and 64QAM) is smaller than in modulation methods in which few bits are transmitted with one symbol (such as binary phase-shift keying (BPSK) and quadrature phase shift keying (QPSK)).
This problem can be solved by, for example, increasing the transmission optical power of the WDM signal to improve the OSNR. When input optical power to an optical transmission path becomes large, however, a penalty caused by a nonlinear optical effect (such as, for example, self-phase modulation or cross-phase modulation) in the optical transmission path also becomes large. Therefore, if transmission power of all optical signals in the WDM signal is increased with the same gain, quality deterioration caused by the nonlinear optical effect (this quality deterioration may be referred to below as the nonlinear penalty) occurs. That is, with the conventional technology, a WDM transmission system in which a plurality of optical signals generated in different modulation methods are included involves a tradeoff between the optical signal-to-noise ratio and the nonlinear penalty.
An object in one aspect of the present disclosure is to suppress, in a WDM transmission system in which a plurality of optical signals generated in different modulation methods are included, quality deterioration in optical signals in these modulation methods.